refactor: remove backward compatibility code for pre-launch baseline

Clean up accumulated backward-compat shims, deprecated wrappers, unused
aliases, and legacy code across the codebase. Since the platform is not
live yet, this establishes a clean baseline.

Changes:
- Delete deprecated middleware/context.py (RequestContext, get_request_context)
- Remove unused factory get_store_email_settings_service()
- Remove deprecated pagination_full macro, /admin/platform-homepage route
- Remove ConversationResponse, InvoiceSettings* unprefixed aliases
- Simplify celery_config.py (remove empty LEGACY_TASK_MODULES)
- Standardize billing exceptions: *Error aliases → *Exception names
- Consolidate duplicate TierNotFoundError/FeatureNotFoundError classes
- Remove deprecated is_admin_request() from Store/PlatformContextManager
- Remove is_platform_default field, MediaUploadResponse legacy flat fields
- Remove MediaItemResponse.url alias, update JS to use file_url
- Update all affected tests and documentation

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/modules/cms/routes/pages/admin.py

@@ -6,7 +6,7 @@ Admin pages for managing platform and store content pages.
 """
 
 from fastapi import APIRouter, Depends, Path, Request
-from fastapi.responses import HTMLResponse, RedirectResponse
+from fastapi.responses import HTMLResponse
 from sqlalchemy.orm import Session
 
 from app.api.deps import get_db, require_menu_access
@@ -22,22 +22,6 @@ router = APIRouter()
 # ============================================================================
 
 
-@router.get("/platform-homepage", include_in_schema=False)
-async def admin_platform_homepage_manager(
-    request: Request,
-    current_user: User = Depends(require_menu_access("platforms", FrontendType.ADMIN)),
-    db: Session = Depends(get_db),
-):
-    """
-    Deprecated: Redirects to platforms page.
-
-    Platform homepages are now managed via:
-    - /admin/platforms → Select platform → Homepage button
-    - Or directly: /admin/content-pages?platform_code={code}&slug=home
-    """
-    return RedirectResponse(url="/admin/platforms", status_code=302)
-
-
 @router.get("/content-pages", response_class=HTMLResponse, include_in_schema=False)
 async def admin_content_pages_list(
     request: Request,